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Old 6th April 2005, 12:53 PM   #1
docali is offline docali  Germany
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Default Distortion analysis mu follower

Hi,
I would like to open the discussion about a surprising subject I discovered during some distortion analysis with PSpice.

The initial intention was to investigate the behaviour of the intermediate resistor during distortion analysis. At first, we have a mu follower circuit with 12ax7 and a coupling capacitor at the lower stage. (see attached file).

...
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Old 6th April 2005, 01:02 PM   #2
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There is nothing surprising with the distortion of the u-Follower into different loads. John Broskie has described it on his page
www.tubecad.com. Look at the SRPP articles.

Reinhard
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Old 6th April 2005, 02:01 PM   #3
EC8010 is offline EC8010  United Kingdom
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Although that circuit is technically a mu-follower, there's hardly any resistance for the upper valve to bootstrap, so the lower valve will see a load of only a couple of hundred k. To gain the advantage of mu-follower action that 2k resistor really ought to be more like 22k or 47k so that the lower valve sees a load resistance much larger than its anode resistance.
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Old 6th April 2005, 02:02 PM   #4
docali is offline docali  Germany
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Ok, let me continue...

If we alter the intermediate resistor R5 then we get for 2nd order distortion:
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Old 6th April 2005, 02:02 PM   #5
docali is offline docali  Germany
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... and for third order distortion:
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Old 6th April 2005, 02:02 PM   #6
docali is offline docali  Germany
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Things change if we remove the coupling capacitor of the lower stage and alter resistor R6:
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Old 6th April 2005, 02:02 PM   #7
docali is offline docali  Germany
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We get a different behaviour because the second order distortion undergo a minimum:
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Old 6th April 2005, 02:02 PM   #8
docali is offline docali  Germany
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The same as with cc, 3rd order distortion decreases with increasing intermediate resistor:
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Old 6th April 2005, 02:02 PM   #9
docali is offline docali  Germany
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If we investigate the numbers, the mu follower without coupling capacitor provides lower absolut distortion values and we have an optimal intermediate resistor because of the second order minimum.
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Old 6th April 2005, 06:08 PM   #10
Yvesm is offline Yvesm  France
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Quote:
Originally posted by docali
If we investigate the numbers, the mu follower without coupling capacitor provides lower absolut distortion values and we have an optimal intermediate resistor because of the second order minimum.
Interesting indeed, but what was the gain and the output level and do you investigate with heavier loads ?
Say 100K or 200K, more practical values for the grid leak of the following stage.

Yves.
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